The present invention pertains to battery cell charging methods and systems. More particularly, the present invention relates to the maintenance charging of lithium-based battery cells, including, for instance, lithium-ion battery cells and lithium-polymer battery cells.
The increasing popularity of consumer electronics in conjunction with the miniaturization of electronic circuits has given rise to a great number of devices which are battery operated. Portable electrical devices including mobile phones, laptop computers, video cameras, and the like, typically rely upon one or more battery cells for electrical power.
Since batteries have a limited capacity, they must periodically be connected to an external charger to be recharged. The conventional units for measuring the capacity of a battery cell are Milliamp Hours (mAh). That is, the dimensional units of mAh are used as a standard measurement for defining the potential power rating, or capacity, of a battery cell. Higher mAh ratings for a battery correlate to longer usage periods for the electronic device being powered by the battery, e.g., a longer talk time and/or standby time for a cellular phone or usage time for a notebook computer.
A number of different types of battery cells are presently in use as power supplies in portable electronic devices. Among the most widespread type of batteries are, nickel-cadmium (Nixe2x80x94Cd), sealed lead acid (SLA), nickel-metal-hydride (NiMH), and, more recently, lithium-ion (Li-ion) and lithium-polymer (Li-polymer). Each of these battery technologies may be characterized by relative advantages and disadvantages.
Nixe2x80x94Cd batteries are a commonly used type of battery cells which are often found in devices requiring relatively large amounts of current (e.g., as required by an electric drill). A primary advantage of Nixe2x80x94Cd battery technology is cost, since Nixe2x80x94Cd batteries tend to be the least expensive type of battery cells. A major disadvantage of Nixe2x80x94Cd battery cells is the xe2x80x9cmemory effect,xe2x80x9d also known as xe2x80x9cvoltage depression,xe2x80x9d which effectively reduces the cell""s capacity if the battery cell is not fully discharged before re-charging. A battery cell impaired by the memory effect appears to be fully charged but lasts only a short time. The memory effect may sometimes be ameliorated by subjecting the battery to a repetitive series of charges and discharges.
Nixe2x80x94MH batteries, a more recent technology than Nixe2x80x94Cd batteries, have attained recent popularity for use in cellular telephones, laptop computers, camcorders, camera flash devices, and like portable consumer electronics devices. Nixe2x80x94MH batteries have a higher charge capacity per unit of weight than Nixe2x80x94Cd batteries, but tend to be more expensive. A major advantage of Nixe2x80x94MH batteries is that they are virtually free of the memory effect which plagues Nixe2x80x94Cd batteries. It is not necessary to fully discharge a Nixe2x80x94MH battery prior to recharging it. However, Nixe2x80x94MH batteries can be damaged by overheating which may occur if the battery is overcharged.
Li-ion batteries have recently gained commercial popularity. Li-ion battery cells typically have a lithium-metal-oxide compound for the positive electrode (the cathode) and a carbon-based compound for the negative electrode (the anode). The Li-ion battery cell becomes charged and discharged as lithium ions migrate between the cathode and the anode, exchanging electrons through doping and de-doping. In short, the migration of electrons produces an electrical current. Li-ion batteries are advantageous over nickel-based batteries in certain respects. For example, Li-ion battery systems have a much higher energy density, as a function of mass. Therefore, for battery systems of equal charge capacities, Li-ion battery systems tend to be lighter and longer lasting than nickel-based battery systems. Another significant advantage of Li-ion battery technology is that Li-ion battery cells do not have the memory effect that exists in other types of nickel-based battery cells, particularly Nixe2x80x94Cd cells.
Li-polymer batteries are another recent entrant in the rechargeable battery market. Li-polymer batteries may be designed to be very thin, and even exhibit some flexibility. Li-polymer batteries are fairly high cost, relative to non-lithium battery technologies.
Another battery technology worth noting in addition to the aforementioned types of batteries are the sealed lead-acid (SLA) batteries. SLA batteries, which are based on well known lead-acid battery technology, are fairly low cost as compared to the other types of batteries mentioned. SLA batteries tend to be heavy, and are often too cumbersome for portable applications.
A conventional mode of charging a Li-ion battery cell involves a two-phase charging process. The charger begins with a charging phase of constant current, and then completes the charging process at a constant voltage. Such a two-phased charging process is termed xe2x80x9cconstant-current, constant-voltagexe2x80x9d (xe2x80x9cCC-CVxe2x80x9d) charging. Conventionally, in the first phase of the charging process, a constant current is applied to the Li-ion battery until the cell approaches its maximum voltage. In the second phase, a constant voltage equal to the maximum cell voltage is applied to the battery until the charge current has decreased to a current cut-off value (e.g., 50 mA, 75 mA, 30 mA). The current cut-off value is an indication of a fully charged battery.
Li-ion batteries have a useful life that typically lasts anywhere from 200 to 1000 charge cycles. Each time a battery is fully charged to its maximum voltage, the useful life of the battery is reduced. It would be useful if the cycle life, that is, the number of charge/recharge cycles, of a battery could be increased. This would extend the useful life of the battery.
Due to the aforementioned drawbacks of conventional charging systems, there is a need in the art for a system and method of charging battery cells so as to extend the useful life of the battery. The present invention utilizes novel charging techniques to address this need by reducing the rate at which the battery capacity diminishes due to repeated charging.
There is also a need for a Li-ion battery charging system and method which does not require additional charging circuitry or logic hardware (i.e., specialized charger logic) which would result in additional cost, weight and volume within a portable battery operated device.
It should be emphasized that the terms xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
An exemplary embodiment of the present invention involves a method of charging a battery in which the battery is repeatedly charged using a first charge voltage during a first time period, and then repeatedly charged using a second charge voltage during a second time period. The second charge voltage is different from the first charge voltage and the second charge voltage diminishes the battery""s capacity at a different rate than the first charge voltage.
Another exemplary embodiment of the present invention involves a method of charging a battery. A determination is made of a first number of cycles during which the battery capacity diminishes by a first amount, a first charge voltage being used to charge the battery. A determination is then made of a second number of cycles during which the capacity of the battery diminishes by a second amount, a second charge voltage being used to charge the battery. The battery is repeatedly charged using said first charge voltage until the battery capacity has diminished by said first amount, and then the battery is repeatedly charged using the second charge voltage. In accordance with a preferred embodiment, the first charge voltage is lower than the second charge voltage. In an alternative embodiment, the first charge voltage could be higher than the second charge voltage.
Another exemplary embodiment of the present invention involves a method of charging a rechargeable battery of a device. In accordance with this exemplary method an expected usage profile for the device is accessed which identifies periods of expected high usage and periods of expected low usage for the device. A charge voltage adjustment scheme is developed based upon the expected usage profile. The battery is then charged to a higher first charge voltage during periods of expected high usage, and charged to a lower second charge voltage during periods of expected low usage.
An exemplary embodiment of the present invention involves a battery charging system which has a variable charge voltage battery charger and a charge voltage decision logic that determines whether a battery is to be charged using a first charge voltage or a second charge voltage. The charge voltage decision logic controls the battery charger to repeatedly charge the battery using the first charge voltage during a first period and using the second charge voltage during a second period. The second charge voltage diminishes the battery capacity at a different rate than the first charge voltage.
Another exemplary embodiment of the present invention involves a battery charging system which has a charge controller, a battery charger, a capacity measurement section, and a logic section that repeatedly charges the battery using a first charge voltage until said capacity measurement section determines that a battery capacity has diminished by a first amount, and uses a second charge voltage. The first charge voltage is higher than the second charge voltage in accordance with this embodiment.
Another exemplary embodiment of the present invention involves a battery charging apparatus which has battery charging circuitry and a controller. The controller controls the battery charger such that is repeatedly charges the battery using a first charge voltage during a first period and using a second charge voltage during a second period of the battery""s cycle life. The second charge voltage is different from the first charge voltage, and the second charge voltage diminishes the battery capacity at a different rate than the first charge voltage.
Another exemplary embodiment of the present invention involves a battery charging apparatus for charging a battery which is repeatedly charged and discharged. In accordance with this exemplary embodiment the battery charger has battery charging circuitry and a controller. The controller causes the battery charging circuitry to charge the battery using a first charge voltage until the controller determines that the battery capacity has diminished by a first amount. Then the controller causes the battery charging circuitry to charge the battery using a second charge voltage until the controller determines that the capacity has diminished by a second amount. In accordance with a preferred embodiment, the first charge voltage is lower than the second charge voltage. In an alternative embodiment, the first charge voltage could be higher than the second charge voltage.
Another exemplary embodiment of the present invention involves a battery charging apparatus which has battery charging circuitry, a controller; and a battery usage database which contains an expected usage profile for the device. The controller accesses the expected usage profile for the device, develops a charge voltage adjustment scheme based upon the expected usage profile, and causes the battery charging circuitry to charge the battery to a first charge voltage during periods of expected high usage and charge the battery to a second charge voltage during periods of expected low usage. In accordance with this embodiment, the first charge voltage is higher than the second charge voltage.
In alternative embodiments of the present invention the current cut-off is adjusted instead of, or in addition to, adjusting the charge voltage.